A device utilizing a piezoelectric phenomenon has been used in a broad field. While miniaturization and power saving of a portable apparatus advance, use of a surface acoustic wave (SAW) device as a filter for RF and IF filter has been increased. An SAW filter has met user's strictly required specifications by improvement of design and production technique, but improvement of characteristics has been nearly limited with increase of a utilized frequency, and a great technical innovation has been required both in miniaturization of electrode formation and securement of stable output.
On the other hand, in a thin film bulk acoustic resonator (hereinafter referred to as FBAR) and stacked thin film bulk acoustic resonators and filters (hereinafter referred to as SBAR) utilizing thickness vibration of a piezoelectric thin film, a thin film mainly constituted of a piezoelectric material, and electrodes for driving it are formed on a thin support film disposed on a substrate, and basic resonance in a gigahertz band is possible. When the filter is constituted of FBAR or SBAR, it is possible to achieve remarkable miniaturization, low loss/broad band operation and integration with a semiconductor integrated circuit, which are expected to be applicable to a future extremely miniature portable apparatus.
A thin film piezoelectric resonator or vibrator such as FBAR, SBAR applied to the resonator, filter or the like utilizing the elastic wave is manufactured as follows.
A substrate film or base film made of a dielectric thin film, an electric conductor thin film, or a laminate thereof is formed on a substrate of a semiconductor single crystal such as silicon, polycrystal diamond formed on a silicon wafer, insulator such as glass, or a constantly elastic metal such as elinvar by various thin film forming methods. A piezoelectric thin film is formed on this substrate film, and an upper structure is further formed if necessary. After forming each layer, or forming all layers, each film is subjected to a physical or chemical treatment to thereby perform patterning and etching. A suspended structure in which a portion positioned under a vibration portion is removed from the substrate is prepared by anisotropic etching based on a wet process, thereafter the obtained structure is separated by the unit of one device if necessary, and accordingly a thin film piezoelectric device is obtained.
For example, a thin film piezoelectric resonator described in JP(A)-58-153412 or JP(A)-60-142607 is manufactured, when a substrate film, a lower electrode, a piezoelectric thin film, and an upper electrode are formed on the upper surface of a substrate, thereafter a via hole is formed by removing a portion of the substrate under a portion constituting the vibration portion from the lower surface of the substrate. When the substrate is made of silicon, a via hole is formed by etching and removing a part of the silicon substrate from the back surface using a heated KOH aqueous solution. Accordingly, a resonator can be prepared having a configuration in which an edge portion of a structure consisted of a layer of a piezoelectric material sandwiched between metal electrodes is supported by a portion around the via hole on the front surface (upper surface) of the silicon substrate.
However, when wet etching is performed using alkali such as KOH, side planes of the via hole are inclined 54.7 degrees with respect to a (100) silicon substrate surface because the etching proceeds in parallel with (111) face, and a distance between centers of adjacent resonators has to be remarkably enlarged. For example, when a resonator having a vibration portion with a plane dimension of about 150 μm×150 μm is constituted on a silicon wafer with thickness of 300 μm, the resonator requires a back-surface etching hole of about 575 μm×575 μm and the distance between the centers of the adjacent resonators is 575 μm or more. This inhibits high density integration of an FBAR resonator. Moreover, when metal electrodes disposed in such a manner as to sandwich a piezoelectric thin film are extended to connect adjacent resonators, electric resistance increases because of longer distance of the metal electrodes. Therefore, there is a problem that insertion loss of the thin film piezoelectric device prepared by combining a plurality of FBAR resonators becomes remarkably large. An acquired amount of final products, that is, the number of thin film piezoelectric resonators formed per unit area on a wafer is limited, and a region of about 1/15 of a wafer area is only utilized for the resonator to produce devices.
A second method of the conventional technique to manufacture thin film piezoelectric resonators such as FBAR, SBAR applied to the thin film piezoelectric device is making of an air bridge type FBAR device as described, for example, in JP(A)-2-13109. Usually, a sacrificial layer is disposed at first, and next a piezoelectric resonator is produced on this sacrificial layer. The sacrificial layer is removed in or near the end of the process, and the vibration portion is formed. Since all processes are performed on the wafer front surface, this method does not require alignment of patterns on the opposite surfaces of the wafer or a large area opening in the back surface of the wafer. In JP(A)-2000-69594, a constitution and a manufacturing method of an air bridge type FBAR/SBAR device using phosphor silicate glass (PSG) as the sacrificial layer are described.
However, in this method, a long complicated step is required. That is, after a series of steps of formation of a hollow in the front surface of the wafer by etching, deposition of the sacrificial layer on the front surface of the wafer by a thermal enhanced chemical vapor deposition (CVD) method, planarization and smoothening of the wafer surface by CMP polishing, and deposition of the lower electrode, the piezoelectric thin film, and the upper electrode and formation of the pattern on the sacrificial layer, a via (hole) extending to the hollow is made, an upper structure deposited on the front surface of the wafer is protected by a resist or the like, and an liquid etching reagent is penetrated through the via hole to thereby remove a sacrificial material from the hollow. Moreover, the number of masks for use in forming the pattern largely increases. As the manufacturing step is long and complicated, the cost of the device is increased, yield of a product drops, which results in further increase of the cost of the device. It is difficult to spread this expensive device as a general-purpose component for a mobile communication apparatus. Since the liquid etching reagent for use in removing sacrificial materials such as phosphor silicate glass (PSG) corrodes the layers of the lower electrode, the piezoelectric thin film, and the upper electrode constituting the upper structure, the materials usable in the upper structure are remarkably limited, Furthermore, there is a serious problem that it is difficult to prepare an FBAR or SBAR structure having a desired dimensional precision.
As piezoelectric materials for the thin film piezoelectric device, aluminum nitride (AlN), zinc oxide (ZnO), cadmium sulfide (CdS), lead titanate (PT(PbTiO3)), lead zirconate titanate (PZT(Pb(Zr, Ti)O3)) and the like are used. Especially, AlN has a high propagation speed of an elastic wave, and is suitable as the piezoelectric material for a thin film piezoelectric resonator and a thin film filter operating in a high-frequency band region.
Since the FBAR and SRAR obtain resonance by propagation of the elastic wave in the thin film, resonance characteristics of the FBAR and SBAR are largely influenced by not only vibration characteristics of the piezoelectric thin film but also those of the electrode layer or the substrate film. Therefore, various restrictions exist from a vibration characteristic aspect with respect to shapes and thicknesses of the electrode layer and the substrate film. For example, when the electrode layer or the substrate film is thickened, there is a problem that effective electromechanical coupling coefficient of the FBAR or SBAR is reduced. On the other hand, when the metal electrode layer is thinned and elongated, conductor loss becomes higher by the increase of electric resistance, and therefore various restrictions are generated in designing the structure of the thin film piezo electric device prepared by combining a plurality of FBARs or SBARs.
The thin film piezoelectric device exerting a sufficient performance in a gigahertz band has not been obtained for the above reason. Therefore, there has been a strong demand for realization of a high-performance thin film piezoelectric device in which all characteristics such as an electromechanical coupling coefficient, acoustic quality factor (Q value), temperature stability of a resonant frequency, and insertion loss of a vibration portion including not only the piezoelectric thin film but also the electrode layer and the substrate film are improved. Especially, the insertion loss is an important parameter which influences the performance in constituting the resonator or the filter, and largely depends on the quality and characteristics of the metal electrode thin film for use.